Method of making hollow castings



June 1, 1954 R. A. KEMPE METHOD OF MAKING HOLLOW CASTINGS Filed Sept. 21, 1949 fZZVEZ-ZLUI" fioberefi. K 8122 03 MW 111L155 Patented June 1, 1954 METHOD OF MAKING HOLLOW CASTINGS Robert A. Kempe, Cleveland, Ohio, assignor to Thompson Products, Inc., Cleveland, Ohio, a

corporation of Ohio Application September 21, 1949, Serial No. 116,869

2 Claims. 1

The present invention relates to a method for making hollow castings.

More particularly, the present invention relates to a method for making turbine buckets, compressor blades, and the like, which have spaced air passages therethrough to provide cooling means for the bucket during the operation of the turbine engine.

In turbo-jet engines, a turbine .operated by burning gases drives a blower furnishing air to the burners. Such turbines operate at very high temperatures, and it is, therefore, very desirable to provide suitable means for cooling the turbine buckets.

An object of the present invention is to provide a method for manufacturing hollow castings having one or more void passages therein at predetermined positions.

Another object of the invention is to provide a method of manufacturing turbine buckets having air passages through their vane portions.

A further object of the invention is to provide a method for manufacturing passaged turbine buckets which controls depth, thickness,andtaper of the passage.

A further object of the present invention is to provide turbine buckets, compressor vanes, and the like, having a high degree of resistance to thermal shock, a high resistance to corrosion, and having passages therethrough permitting the circulation of a cooling fluid.

In essence, the present invention comprises a method for producing hollow castings having voids extending in predetermined relationship through the casting by forming a body of molybdenum, which may be molybdenum Wire, strip molybdenum or molybdenum tubes, into the configuration desired in the voids of the final casting, casting the body metal about the prearranged molybdenum body, and finally heating the resulting structure in an oxidizing atmosphere at a temperature suflficient to form molybdenum trioxide from the molybdenum. When molybdenum is heated in air at a temperature of about 1,500 E, the trioxide of molybdenum is formed. This trioxide sublimes so that the oxide is volatilized almost immediately upon formation, resulting in the disintegration of the molybdenum. It is this characteristic of molybdenum that I employ to remove the molybdenum from the body metal, thus leaving voids in the structure corresponding to the position of the original molybdenum network.

Since, under certain conditions, molybdenum may be taken into solution in the body metal during casting, means are provided within the scope of the invention for preventing such solution, these means comprising coating the molybdenum wire with a particular type of ceramic composition which protects the body metal during the period when the molybdenum trioxide is being volatilized and prevents such solution.

In addition, since molybdenum trioxide is a powerful oxidizing agent, I have provided means for protecting the outer surfaces of the body metal during the time when the molybdenum trioxide is being volatilized from the body metal.

The advantages of the presently proposed process are many fold. For one, extremely fine ducts may be formed in the turbine bucket. Since molybdenum is quite malleable, molybdenum.-

wire of a very small diameter may be provided within the body structure of the turbine bucket, so that upon volatilization, very fine diameter air passages result.

In addition, another important advantage of the present procedure is that the position of the ducts may be located precisely within the article. By providing cooling ducts parallel to and quite close to the leading edge of a turbine bucket, the tendency of the bucket to overheat along its leading edge is substantially reduced.

Another advantage of the method herein proposed lies in the fact that the ducts produced in the cast article may have any configuration and need not be continuous. For example, it may be desirable in some cases to provide tapered air passages which terminate inside the body structure itself. This type of structure is also easily produced by the practice of the present invention.

The complete process for the production of a turbine bucket using the principles of the present invention will be presently described, but it will be appreciated by those skilled in the art that the invention is in no way restricted to the production of such turbine buckets or other parts for gas turbine engines, but will find general applicability in casting procedures wherein it is desired to produce hollow castings having one or more void passages therein of predetermined length and position.

The first step of the procedure consists in providing molybdenum wire, or strips, and forming the same into the configuration of the void passages which are to be produced in the finished turbine bucket. It is generally preferable to coat the molybdenum Wire with a ceramic composition to prevent the molybdenum from migrating into the body metal during theoxidization proce- Per cent Alumina 20 Silica 60 Calcium oxide 15 Boric oxide This glass composition and other similar glass compositions have the property of high resistance to thermal shock, and also have a coefiicient of thermal expansion which is close to that of molybdenum itself. In addition, the compositions show good adherence to molybdenum.

The molybdenum wires in the network desired for the voids of the final bucket preferably are held in place by having their ends encased in a refractory ceramic block. This block serves not only to hold the molybdenum strands in correct alignment, but also may serve as a bottom closure for the subsequent molding steps.

After arranging the molybdenum wire in the proper configuration, a pattern of a low-melting substance is produced about the molybdenum wire strands in the shape of the bucket which is to be cast around the strands. This low-melting substance is preferably wax, although substances such as mercury, lead, and other lowmelting metals or alloys may be used as well. After the low-melting pattern has been set, the assembly is invested in an investment mold which is preferably a ceramic mold of the type commonly used in precision casting. There, the wax pattern is melted out and a suitable body metal is cast about the molybdenum network. The only qualification of a body metal limiting its use in the present invention is that it be able to withstand the temperatures in the vicinity of 1,500 F. wihout substantial corrosion or without substantial loss of hot strength. Suitable alloys for casting turbine buckets are well known in the art as, for example, .Stellites, Vitallium alloys which are stainless alloys of cobalt, chromium and molybdenum and various cobalt base alloys containing chromium, tungsten, and carbon may also be used.

After the bucket has been cast about the molybdenum network, the mold is removed. At this stage, it is often desirable to provide the outer surface of the turbine bucket body with a corrosion-resistant coating to protect the same during the subsequent oxidization step. Three eminently suitable materials for coating purposes in this connection are silicon, aluminum and zirconium. These coatings have the ability to form strongly adhering, corrosion-resistant about the bucket body.

After the casting procedure, the assembly comprising the cast turbine bucket body containing the molybdenum wire is subjected to an oxidization operation for a time and at a temperature suificient to form molybdenum trioxide from the molybdenum wire and to volatilize the molybdenum trioxide. Normally, temperatures of 1,500 F. and above, will be employed during this step. The upper limit for this oxidizing temperature is fixed by the temperature at which deterioration of the properties of the body metal begins to occur. To make the atmosphere more strong- 4 1y oxidizing, oxygen or water vapor may be added to an air atmosphere during this treatment.

After the molybdenum has been completely removed by volatilization, any ceramic material which was added as a protection to the body metal may be removed from the void passages formed in the body by simply shaking the article until the ceramic material drops out.

The bucket at this stage will have air passages therethrough corresponding to the positions of the original molybdenum wire. The cast bucket can then be. conveniently machined or brought down to finished dimensions, if necessary.

Where it is desired to make relatively large hollow passages within a cast article, the above procedure may be modified by substituting hollow molybdenum tubes for the molybdenum wire. Since molybdenum has a rather low coefficient of expansion, there is a possibility that the molybdenum tubes will buckle during casting. To overcome this, and provide internal support against the pressure of the liquid metal durin pouring, the tubes may be filled with a loosely compacted ceramic material of the type indicated above, which may be completely forced out of the passages after the casting procedure.

The procedure described above is illustrated in the drawing, in which:

Figure 1 is a view in perspective of the molybdenum wire network as originally formed;

Figure 2 is a perspective view of the assembly after the wax pattern has been formed thereon;

Figure 3 is a cross-sectional plan vieu of a ceramic mold assembly containing the molybdenum wire network;

Figure 4 is a perspective view, with parts broken away, to show the internal void passages of a cast turbine bucket; and

Figure 5 is a cross-sectional view taken substantially along the line VV of Figure 4.

As shown on the drawing:

As shown in Figure 1, the first step of the procedure is to form a network of individual molybdenum wires H] in the alignment desired for the 5 air ducts to be formed in the turbine bucket.

The ends ofthe individual wires it! are anchored in a refractory ceramic block II in their proper alignment. The individual molybdenum wires H) are preferably slightly longer than the actual length of the air ducts to be formed so that the wire ends can be anchored in the mold.

The next step is to cast a low-melting pattern about the network of molybdenum wires l0, and such a pattern is illustrated in Figure 2. It will be seen that this pattern comprises a vane portion l2 and a base or root portion l3. Next, the assembly shown in Figure 2 is invested in a ceramic investment mold [4. Here, the wax or other low-melting material is melted out, leaving the individual wires Ill supported within the ceramic mold [4 by the refractory ceramic block H which can also serve as a bottom closure for the mold it. If the ends of the wires project beyond the pattern they will be anchored in the mold.

A suitable body metal is next cast around the molybdenum network. After the casting has set, the entire casting is submitted to an oxidation treatment described previously where the molybdenum is removed from the body metal by sublimation. Prior to the oxidation procedure, the ceramic block H is trimmed from the assembly. The oxidation step thus leaves a bucket body I5 on a base I6 and having air passages ll extending through the base portion l6 and into the leading and trailing edges of the vane portion of the turbine bucket. Thus, when the turbine bucket is mounted in the turbine assembly, compressed air may be forced through the passages [7, thus cooling the interior of the bucket body as well as the leading and trailing edges.

From the foregoing, it will be apparent that I have herein provided a method for producing hollow castings, and more particularly, a method for providing air passages within the body structure of a turbine bucket. The procedure given above is adaptable to a large variety of precision casting methods where it is desired to provide a casting with void passages of definite configuration, size and shape.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concept of the present invention.

I claim as my invention:

1. The method of manufacturing castings having void passages therethrough in predetermined configuration which comprises forming a molybdenum core coated with a ceramic material into a network having the configuration required in the void passages, casting a heat-resistant body metal about the coated core, heating the assembly with the molybdenum core exposed in an oxidizing atmosphere to a temperature sufficient to oxidize the molybdenum into molybdenum trioxide and to volatilize the molybdenum trioxide formed, and subsequently removing said ceramic material from the body metal.

2. The method of making castings having void passages therethrough in a predetermined configuration which comprises forming molybdenum wire having a coating comprising vitrified calcium sages, casting a heat-resistant body metal about the molybdenum network, heating the assembly and exposing said wire to an oxidizing atmosphere of a temperature sufiicient to oxidize the molybdenum into molybdenum trioxide and to volatilize the molybdenum trioxide formed, and subsequently removing said vitrified silicates from the body metal.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,416,412 Pack May 16, 1922 1,465,472 Hansen Aug. 21, 1923 1,544,930 Pack July 7, 1925 1,710,534 Field Apr. 23, 1929 1,988,861 Thorausch et a1. Jan. 22, 1935 2,074,007 Wissler Mar. 16, 1937 2,085,324 Linclner June 29, 1937 2,204,123 Collins June 11, 1940 2,362,507 Steinbock et al. Nov. 14, 1944 2,362,875 Zahn Nov. 14, 1944 2,368,295 Goran Jan. 30, 1945 2,368,296 Goran Jan. 30, 1945 2,373,405 Lowit Apr. 10, 1945 2,388,299 Thielemann Nov. 6, 1945 2,499,977 Scott Mar, 7, 1950 2,510,735 Badger June 6, 1950 FOREIGN PATENTS Number Country Date 588,113 Great Britain May 14, 1947 608,766 Great Britain Sept. 21, 1948 238,186 Switzerland Nov. 1, 1945 245,674 Germany Apr. 15, 1912 OTHER REFERENCES Metal Progress, May 1948, vol. 53, page 684. 

1. THE METHOD OF MANUFACTURING CASTINGS HAVING VOID PASSAGES THERETHROUGH IN PREDETERMINED CONFIGURATION WHICH COMPRISES FORMING A MOLYBDENUM CORE COATED WITH A CERAMIC MATERIAL INTO A NETWORK HAVING THE CONFIGURATION REQUIRED IN THE VOID PASSAGES, CASTING A HEAT-RESISTANT BODY METAL ABOUT THE COATED CORE, HEATING THE ASSEMBLY WITH THE MOLYBDENUM CORE EXPOSED IN AN OXIDIZING ATMOSPHERE TO A TEMPERATURE SUFFICIENT 